Recently, we discovered that a secreted chitinase of Legionella pneumophila, the agent of Legionnaires'disease, promotes the persistence of the bacterium in the lungs of experimentally infected mice. In vitro experiments then documented that the L. pneumophila chitinase (ChiA) promotes growth in lung epithelial cells but not in macrophages. Taken together, these data are the first demonstration of a chitinase, a type of enzyme that is traditionally viewed as only being relevant in chitin-containing environmental niches, acting like a virulence factor. Thus, we hypothesize that bacterial chitinases represent a new class of virulence factor. To explore this hypothesis, we will further investigate the role of chitinase in L. pneumophila intracellular infection of lung cells as well as test for the first time the importance of a chitinase in the virulence of a second intracellular pathogen, Listeria monocytogenes. More specifically, we will determine the intracellular location of and trafficking patterns influenced by ChiA and begin to ascertain the effects of ChiA on the lung inflammatory response. For the Listeria experiments, new chitinase mutants will be constructed and then, after confirmation of their loss of chitinase activity and their in vitro growth characteristics, tested in murine models of listeriosis. The proposed studies will i) increase our understanding of the pathogenesis of Lp, which is an important public health concern within the US and throughout the world, ii) examine for the first time the role of chitinase in the pathogenesis of L. monocytogenes, an organism that is also an important public health concern, iii) expand our appreciation of bacterial intracellular infection and the novel role that a chitinase can have in it, iv) have implications for numerous other environmental pathogens that also express chitinases, and iv) offer potential new targets for disease diagnosis, treatment, or prevention. PUBLIC HEALTH RELEVANCE: We recently discovered that the secreted chitinase of Legionella pneumophila promotes bacterial persistence in the lungs of experimentally infected mice. From this novel finding, we hypothesize that microbial chitinases, which have traditionally been viewed as being relevant only in environmental niches, actually represent a new class of virulence factor and therefore are potential new targets for antimicrobial therapy. To explore this hypothesis, we will further investigate the role of chitinase in L. pneumophila intracellular infection of lung cells as well as test for the first time the importance of chitinases in the virulence of a second intracellular pathogen, Listeria monocytogenes.